I know Firewire is faster than USB (especially FW800), and I've heard that USB recording interfaces can have some lag issues. With all the USB interfaces on the market though (a much better selection than Firewire ones), I have to wonder if it really matters. Does anyone have actual experience with multitracking through Firewire vs USB? Also, do factors like your computer's RAM have much to do with it?

I use a Line 6 UX2 (USB) for guitar, bass, keys or vocals and for that it is outstanding. But USB really only works well for simultaneous recording of two or four channels at the most. Your kit may use seven or more mics for recording and you need firewire for that. Here is a good quality firewire interface for recording drums that is still affordable: